Non-volatile semiconductor memory device

ABSTRACT

A non-volatile semiconductor memory device including: a NAND string having multiple memory cells connected in series and first and second select gate transistors disposed on the both ends; word lines coupled to the memory cells; and first and second select gate lines coupled to the first and second select gate transistors, wherein a data read mode is defined by the following bias condition: a selected word line is applied with a read voltage; one adjacent to the selected word line within first unselected word lines disposed on the first select gate line side is applied with a first read pass voltage while the others are applied with a second read pass voltage lower than the first read pass voltage; and second unselected word lines disposed on the second select gate line side are applied with a third read pass voltage higher than the first read voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims the benefit of priority from theprior Japanese Patent Application No. 2008-032820, filed on Feb. 14,2008, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a non-volatile semiconductor memory device(EEPROM), specifically to a data read method of a NAND-type flashmemory, in which multiple memory cells are connected in series toconstitute a NAND string.

2. Description of the Related Art

A NAND-type flash memory is well known as one of EEPROMs, which iseasily achieving a high integration. In a data read mode of theNAND-type flash memory, unselected memory cells in a NAND string aremade to operate as pass transistors, and it will be detected the on/offstate of a selected cell. That is, the selected cell is applied with aread voltage necessary for judging the cell threshold distribution, andthe remaining unselected cells are applied with a read pass voltage thatturns on cells without regard to cell data, whereby data is judged inaccordance with whether the precharged bit line is discharged or not.

As the integration of the above-described NAND-type flash memoryprogresses, the reliability of read data becomes problematic.Specifically, the capacitive coupling between adjacent cells influencesthe read operation. For example, it will be generated such a situationthat the floating gate potential in an unselected cell adjacent to aselected cell is not raised sufficiently, and this unselected cellbecomes disable to carry a sufficient cell current as a pass transistor.Especially, unselected cells disposed on the lower course side of theselected cell (i.e., on the source line side of the selected cell) areproblematic because these are set in a high channel resistance state dueto negative feedback serving for lowering the cell current. This cellcurrent decrease at a data read time causes the erroneous read andincreasing of the sensing time.

With respect to highly integrated NAND-type flash memories, there havealready been some proposals, in which the influences of capacitivecoupling between adjacent cells are considered. For example, refer toJP-A-2002-133888. In this application, there is shown that an unselectedword line adjacent to a selected word line is applied with a read passvoltage set to be lower than that of the other unselected word lines ata write verify-read time.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided anon-volatile semiconductor memory device including:

a NAND string, in which a plurality of memory cells are connected inseries and first and second select gate transistors are disposed on theboth ends for coupling them to a bit line and a source line,respectively;

a plurality of word lines coupled to the respective control gates of thememory cells; and

first and second select gate lines coupled to the gates of the first andsecond select gate transistors, respectively, wherein

a data read mode is defined by the following bias condition: a selectedword line is applied with a read voltage; one adjacent to the selectedword line within first unselected word lines disposed on the firstselect gate line side of the selected word line is applied with a firstread pass voltage while the others are applied with a second read passvoltage lower than the first read pass voltage; and second unselectedword lines disposed on the second select gate line side of the selectedword line are applied with a third read pass voltage higher than thefirst read voltage.

According to another aspect of the present invention, there is provideda non-volatile semiconductor memory device including:

a NAND string, in which a plurality of memory cells are connected inseries and first and second select gate transistors are disposed on theboth ends for coupling them to a bit line and a source line,respectively;

a plurality of word lines coupled to the respective control gates of thememory cells; and

first and second select gate lines coupled to the gates of the first andsecond select gate transistors, respectively, wherein

a data read mode is defined by the following bias condition: a selectedword line is applied with a read voltage; one adjacent to the selectedword line within first unselected word lines disposed on the firstselect gate line side of the selected word line is applied with a firstread pass voltage while the others are applied with a second read passvoltage lower than the first read pass voltage; and second unselectedword lines disposed on the second select gate line side of the selectedword line are applied with the first read pass voltage.

According to still another aspect of the present invention, there isprovided a non-volatile semiconductor memory device including:

a NAND string, in which a plurality of memory cells are connected inseries and first and second select gate transistors are disposed on theboth ends for coupling them to a bit line and a source line,respectively;

a plurality of word lines coupled to the respective control gates of thememory cells; and

first and second select gate lines coupled to the gates of the first andsecond select gate transistors, respectively, wherein

a data read mode is defined by the following bias condition: a selectedword line is applied with a read voltage; one adjacent to the selectedword line within first unselected word lines disposed on the firstselect gate line side of the selected word line is applied with a firstread pass voltage while the others are applied with a second read passvoltage lower than the first read pass voltage; and one adjacent to theselected word line within second unselected word lines disposed on thesecond select gate line side of the selected word line are applied withthird read pass voltage higher than the first read pass voltage whilethe others are applied with the first read pass voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a voltage distribution in a NAND string at a read time inaccordance with embodying mode 1.

FIG. 2 shows a voltage distribution in a NAND string at a read time inaccordance with embodying mode 2.

FIG. 3 shows a voltage distribution in a NAND string at a read time inaccordance with embodying mode 3.

FIG. 4 shows a voltage distribution in a NAND string in case the wordline nearest to the bit line side select gate line is selected.

FIG. 5 shows a voltage distribution in a NAND string in case the wordline nearest to the source line side select gate line is selected.

FIG. 6 shows a voltage distribution in a NAND string at a read time inaccordance with embodying mode 4.

FIG. 7 shows a voltage distribution in a NAND string at a read time inaccordance with embodying mode 5.

FIG. 8 shows a memory core of a NAND-type flash memory in accordancewith the present invention.

FIG. 9 shows the voltage waveforms in a normal data read mode in theflash memory.

FIG. 10 shows the binary data threshold distribution of the flashmemory.

FIG. 11 shows the capacitive coupling situation between cells in a NANDstring.

FIG. 12 shows a voltage distribution in a NAND string at a normal readtime.

FIG. 13 shows a data write sequence of the flash memory.

FIG. 14 shows the functional block configuration of the flash memory.

FIG. 15 shows another embodiment applied to a digital still camera.

FIG. 16 shows the internal configuration of the digital still camera.

FIGS. 17A to 17J show other electric devices to which the embodiment isapplied.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Illustrative embodiments of this invention will be explained withreference to the accompanying drawings below.

FIG. 8 shows an equivalent circuit of a memory core of a NAND-type flashmemory in accordance with an embodiment. Memory cell array 1 has NANDstrings (i.e., NAND cell units) 5 arranged therein, in each of which aplurality of memory cells are connected in series. In this example, aNAND string has thirty two memory cells M0-M31 connected in series. Oneend of the NAND string is coupled to a bit line BL via select gatetransistor SG1; and the other to a common source line CELSRC via selectgate transistor SG2.

Control gates of the memory cells M0-M31 are coupled to word linesWL0-WL31, respectively; and gates of the select gate transistors SG1 andSG2 to select gate lines SGD and SGS, respectively.

Disposed for selectively driving the word lines WL0-WL31 and select gatelines SGD and SGS is a row decoder 2. Bit lines are coupled to the senseamplifiers S/A constituting a page buffer 3. A set of NAND stringssharing word lines is defined as a block 4, which serves as an eraseunit. Multiple blocks are arranged in the bit line direction.

FIG. 9 shows voltage waveforms used in a read operation of a usualNAND-type flash memory. It is assumed here that the binary datathreshold distribution shown in FIG. 10 is used. As shown in FIG. 9, bitline precharge is performed (timing T0) under the condition that aselected word line is applied with read voltage Vcg (e.g., 0.4V);unselected word lines are applied with read pass voltage Vread (e.g.,5V); and select gate line SGD on the bit line side is applied withvoltage Vsg for turning on the select gate transistor (e.g., 4V), sothat the bit line is precharged up to a certain bit line voltage VBL(e.g., 1V).

As shown in FIG. 10, the read voltage Vcg is set between the cellthreshold distributions of data “0” and “1”, i.e., set in such a statethat cell current flows in a data “1” cell (erased cell) while cellcurrent does not flow in a data “0” cell (programmed or written cell).Read pass voltage Vread is set at a level necessary for sufficientlyturning on cells without regard to cell data.

After the bit line precharge, the select gate line SGS on the sourceline side is applied with Vsg for turning the select gate transistor(timing T1). As a result, if the selected cell is data “1”, the NANDstring becomes conductive, and the bit line is discharged while if theselected cell is data “0”, the bit line will not be discharged. Detectwhether the bit line is discharged or not (or whether the amount of thebit line discharge is large or not), and it becomes possible to judgethe cell data.

There is a problem in the above-described read operation as follows. Asthe high integration progresses, the distance between word lines isnarrowed. On the other hand, there is a limitation for thinning the gateinsulating film when taking into consideration of the reliability ofcell properties. Therefore, capacitive coupling increases betweenadjacent word lines, or between word lines and adjacent cells' floatinggates.

FIG. 11 shows the situation of capacitive coupling between adjacent wordlines in a NAND string. A floating gate (FG) is coupled to a controlgate (CG) (i.e., word line) disposed above it with capacitance C1, andcoupled to the cell channel with capacitance C2. In addition, a floatinggate is coupled to the adjacent floating gate and control gatethereabove with capacitances C3 and C4, respectively. If capacitances C3and C4 become large as being not ignorable in comparison withcapacitances C1 and C2, these influence the cell data read operation.

FIG. 12 shows voltage distributions of word lines WL (i.e., controlgates CG) and floating gates FG in a NAND string in case the wholeunselected word lines are applied with read pass voltage Vread.

Since the voltage of the floating gate FG is primarily defined by thecapacitance division between the control gate CG and the cell channel,the floating gate FG of the unselected cell will be set at Vread′(<Vread). However, considering an unselected cell disposed on the bitline side or on the source line side to be adjacent to the selected cell(such the unselected cell will be referred to as a “adjacent-unselectedcell”, hereinafter), the floating gate FG is lowered in levelsubstantially due to the capacitive coupling from the selected cell.While, the voltage of the floating gate of the selected cell will beboosted under the influence of the adjacent-unselected cell. Therefore,there is generated such a situation that the adjacent-unselected cell isnot turned on sufficiently, and the selected cell also is not turned onsufficiently even if the cell data is “1”.

In this embodiment, the read pass voltage applied to the unselected wordlines is suitably set, and the reliability of the data read operation isimproved. In correspondence to the above-described “adjacent-unselectedcell”, the unselected word line coupled to this cell will be referred toas an “adjacent-unselected word line” hereinafter.

The details of the embodiment will be explained below.

Embodying Mode 1

FIG. 1 shows a voltage distribution in a NAND string at a data read timein accordance with embodying mode 1. The selected word line is appliedwith read voltage Vcg set between cell data threshold distribution to beread and the adjacent threshold distribution. Unselected word linesdisposed on the bit line (BL) contact side (i.e., on the select gateline SGD side) of the selected word line are set as follows: oneadjacent-unselected word line adjacent to the selected word line isapplied with read pass voltage Vread1; and the other unselected wordlines are applied with read pass voltage Vread2 set to be lower thanVread1. All unselected word lines disposed on the source line (CELSRC)contact side (i.e., on the select gate line SGS side) are applied withread pass voltage Vread3 set to be higher than Vread1.

This will be explained in other words as follows. Assuming that readpass voltage Vread2 is equal to Vread usually used as a uniform readpass voltage as explained with reference to FIG. 12, read pass voltageVread1 higher than Vread is applied to the adjacent-unselected word lineon the select gate line SGD side, and read pass voltage Vread3 higherthan Vread1 is applied to all unselected word lines on the select gateline SGS side.

With these read pass voltages applied, the voltage drop of the floatinggate under the adjacent-unselected word line due to capacitive couplingfrom the selected cell is suppressed, so that the adjacent-unselectedcell will be turned on sufficiently. Especially, as a result of thatunselected word lines on the select gate line SGS side are applied withsufficiently high read pass voltage Vread3, it is suppressed such a backgate effect (i.e., negative feed-back effect) that the source potentialof the selected cell is boosted due to the channel resistance on thesource side, and this makes possible to carry a sufficient cell currentin the NAND string.

As a result, the read margin increases, i.e., the reliability of dataread operation will be improved, and it becomes possible to shorten thesensing time.

As shown in FIG. 4, when word line WL31 nearest to the select gate lineSGD is selected, all unselected word lines are applied with read passvoltage Vread3 shown in FIG. 1. As a result, the channel resistance onthe source side of the selected cell is sufficiently lowered, so thatthe cell current may be made to be sufficiently large.

As shown in FIG. 5, when word line WL0 nearest to the select gate lineSGS is selected, adjacent-unselected word line is applied with read passvoltage Vread1; and the other unselected word lines with read passvoltage Vread2 lower than Vread1. This relationship is the same as thatshown in FIG. 1. As a result, the floating gate voltage drop of theadjacent-unselected cell is suppressed, and a sufficiently large cellcurrent flows.

Embodying Mode 2

FIG. 2 shows a voltage distribution in a NAND string at a data read timein accordance with embodying mode 2. The selected word line is appliedwith read voltage Vcg set between cell data threshold distribution to beread and the adjacent threshold distribution. Unselected word linesdisposed on the bit line (BL) contact side (i.e., on the select gateline SGD side) of the selected word line are set as follows: oneadjacent-unselected word line adjacent to the selected word line isapplied with read pass voltage Vread1; and the other unselected wordlines are applied with read pass voltage Vread2 lower than Vread1. Allunselected word lines disposed on the source line (CELSRC) contact side(i.e., on the select gate line SGS side) are applied with read passvoltage Vread1.

This will be explained in other words as follows. Assuming that readpass voltage Vread2 is equal to Vread usually used as a uniform readpass voltage as explained with reference to FIG. 12, read pass voltageVread1 higher than Vread is applied to the adjacent-unselected word lineon the select gate line SGD side and all unselected word lines on theselect gate line SGS side.

As a result, as similar to the embodying mode 1, a sufficiently largecell current is secured, and the reliability of data read operation willbe achieved. It is an advantageous effect that the number of read passvoltages is less than that in the embodying mode 1.

Embodying Mode 3

FIG. 3 shows a voltage distribution in a NAND string at a data read timein accordance with embodying mode 3. The selected word line is appliedwith read voltage Vcg set between cell data threshold distribution to beread and the adjacent threshold distribution. Unselected word linesdisposed on the bit line (BL) contact side (i.e., on the select gateline SGD side) of the selected word line are set as follows: oneadjacent-unselected word line adjacent to the selected word line isapplied with read pass voltage Vread1; and the other unselected wordlines are applied with read pass voltage Vread2 lower than Vread1.

By contrast, unselected word lines disposed on the source line (CELSRC)contact side (i.e., on the select gate line SGS side) of the selectedword line are set as follows: one adjacent-unselected word line adjacentto the selected word line is applied with read pass voltage Vread3higher than Vread1; and the other unselected word lines are applied withread pass voltage Vread1.

What is different from the embodying mode 1 shown in FIG. 1 is asfollows: in contrast to that all unselected word line on the select gateline SGS side are applied with read pass voltage Vread3 in the embodyingmode 1, only adjacent-unselected word line is applied with Vread3 inthis embodying mode 3. According to this embodying mode 3, it is able tosuppress the channel resistance increase of the adjacent-unselectedcell, which has the largest influence to the source resistance of theselected cell in the whole unselected cells on the select gate line SGSside, so that the read property may be sufficiently improved.

Embodying Mode 4

FIG. 6 shows an example in which dummy cells DC1 and DC2 are disposedadjacent to the select gate transistor SG1 and SG2, respectively, on theboth ends of the NAND string. In this case, it is assumed that a NANDstring has 64 memory cells M0-M63. Dummy cells DC1 and DC2 are formed assimilar to the normal cells and disposed for preventing the adjacentmemory cells M63 and M0, respectively, from being erroneously written.These dummy cells are not used for data storing, but it is required ofthese dummy cells to serve as pass transistors at a read time.Therefore, control gates of these dummy cells DC1 and DC2 are coupled todummy word lines DWLD and DWLS, respectively, which are disposed inparallel with word lines.

The read bias scheme shown in FIG. 6 is basically the same as that ofthe embodying mode 1 shown in FIG. 1. Dummy word line DWLS on the selectgate line SGS side is applied with read pass voltage Vread3 as well asthe unselected word line WL0 adjacent to DWLS while dummy word line DWLDon the select gate line SGD side is applied with read pass voltageVread2 as well as the unselected word line WL63 adjacent to DWLD.

Dummy cells DC1 and DC2 are not used for storing data, but these areused in an erase mode. Therefore, although the channel resistancesthereof do not become problematic, securing sufficient cell current aswell as the embodying mode 1, the reliability of data read will beimproved.

The schemes in the embodying modes 2 and 3 explained with reference toFIGS. 2 and 3, respectively, may also be adaptable for the NAND stringwith the above-described dummy cells. That is, in addition to the biasconditions shown in FIGS. 2 and 3, dummy word line DWLS adjacent to theselect gate line SGS is applied with read pass voltage Vread1, and dummyword line DWLD adjacent to the select gate line SGD is applied with readpass voltage Vread2 (not shown).

Embodying Mode 5

FIG. 7 shows another example, in which dummy cells DC1 and DC2 aredisposed adjacent to the select gate transistor SG1 and SG2,respectively, on the both ends of the NAND string like that shown inFIG. 6.

As described above, dummy cells are made to operate in the erased state.Therefore, even if these are erroneously written due to GIDL (GateInduced Drain Leakage) current, the threshold voltage does not becometoo high. By reason of this, it is allowed to apply a relatively lowread pass voltage to the dummy word lines DWLD and DWLS.

In the example shown in FIG. 7, dummy word line DWLS on the select gateline SGS is applied with read pass voltage Vread1 lower than Vread3applied to unselected word line WL0 adjacent to the dummy word lineDWLS; and dummy word line DWLD on the select gate line SGD is appliedwith read pass voltage Vread4 lower than Vread2 applied to unselectedword line WL63 adjacent to the dummy word line DWLD.

According to this embodying mode, although the read pass voltages becomenumerous, the erroneous writing stress in the dummy cells is suppressed,and the reliability of data read will be improved.

This embodying mode may also be adapted to the schemes explained withreference to FIGS. 2 and 3 in case the NAND string has dummy cells assimilar to that shown in FIG. 7. That is, in addition to the biasconditions shown in FIGS. 2 and 3, dummy word line DWLS adjacent to theselect gate line SGS is applied with read pass voltage Vread1, and dummyword line DWLD adjacent to the select gate line SGD is applied with readpass voltage Vread4 (not shown).

So far, various word line bias situations in the normal data read modehave been explained. The present invention may also be adaptable for theverify-read operation in a data write mode.

For example, a data write sequence has, as shown in FIG. 13, one pagewrite data loading step (S1); write pulse applying step (S2);verify-reading step (S3); and verify-checking step (S4). If there isinsufficiently written cell, write voltage is stepped-up, and the writestep will be repeated until when the verify passes.

At the write-verify read time, as shown in FIG. 10, verify-read voltageVv different from the read voltage Vcg in the normal read mode is usedfor verifying the lower limit value of the data “0” thresholddistribution. Unselected word lines are applied with a read passvoltage(s) as similar to the normal read case. Therefore, only replacingthe read voltage Vcg with the verify-read voltage Vv, theabove-described embodying modes are adaptable to the write-verify readoperation.

Further, the present invention may be adapted to such a read scheme thatcell current is made to flow from the common source line CELSRC with Vddapplied to the bit line BL. In this case, a selected word line isapplied with, for example, the power supply voltage Vdd; and unselectedword lines with read pass voltage Vread. Considering the read passvoltage Vread as well as in the above-described embodying modes, thesame effect as the above-described embodying modes will be obtained.

Additionally, the present invention may be adapted to the NAND-typeflash memory with a multi-level data storage scheme only taking intoconsideration the absolute values of the read voltage and read passvoltage.

FIG. 14 shows a functional block configuration of the NAND-type flashmemory in accordance with the above-described embodying modes 1-5.

Memory cell array 1, row decoder 2 and sense amplifier circuit 3constitute the memory core as described above. Sequence controller(i.e., internal control circuit) 7 controls read, write and eraseoperations of the memory core.

The page buffer 3 has data hold circuits for one page, which is used fordata reading and data writing for each one page of the memory cell array1. One page read data of the page buffer 3 are sequentially selected bya column decoder 4 to be output to external I/O terminals through an I/Obuffer 9. Write data supplied from the I/O terminals are selected by thecolumn decoder 4 to be loaded in the page buffer 3.

Address “Add.” and command “Com.” are input through I/O buffer 9 to betransferred to address buffer 5 and controller 7, respectively. Rowaddress and column address are transferred to the row decoder 2 andcolumn decoder 4.

Logic controller 6 controls for command inputting, address inputting anddata inputting/outputting in response to external control signals suchas chip enable signal /CE, command latch enable signal CLE, addresslatch enable signal ALE, write enable signal /WE, read enable signal /REand the like. Read and write operations are performed in accordance withcommand “Com.”. In response to the command, sequence controller 7executes read control and sequence control of data write and data erase.

A high voltage generator 8 is controlled by the controller 7 to outputpredetermined voltages necessary for many kinds of operations.

[Application Devices]

As an embodiment, an electric card using the non-volatile semiconductormemory devices according to the above-described embodying modes 1-5 ofthe present invention and an electric device using the card will bedescribed bellow.

FIG. 15 shows an electric card according to this embodiment and anarrangement of an electric device using this card. This electric deviceis a digital still camera 101 as an example of portable electricdevices. The electric card is a memory card 61 used as a recordingmedium of the digital still camera 101. The memory card 61 incorporatesan IC package PK1 in which the non-volatile semiconductor memory deviceor the memory system according to the above-described embodiments isintegrated or encapsulated.

The case of the digital still camera 101 accommodates a card slot 102and a circuit board (not shown) connected to this card slot 102. Thememory card 61 is detachably inserted in the card slot 102 of thedigital still camera 101. When inserted in the slot 102, the memory card61 is electrically connected to electric circuits of the circuit board.

If this electric card is a non-contact type IC card, it is electricallyconnected to the electric circuits on the circuit board by radio signalswhen inserted in or approached to the card slot 102.

FIG. 16 shows a basic arrangement of the digital still camera. Lightfrom an object is converged by a lens 103 and input to an image pickupdevice 104. The image pickup device 104 is, for example, a CMOS sensorand photoelectrically converts the input light to output, for example,an analog signal. This analog signal is amplified by an analog amplifier(AMP), and converted into a digital signal by an A/D converter (A/D).The converted signal is input to a camera signal processing circuit 105where the signal is subjected to automatic exposure control (AE),automatic white balance control (AWB), color separation, and the like,and converted into a luminance signal and color difference signals.

To monitor the image, the output signal from the camera processingcircuit 105 is input to a video signal processing circuit 106 andconverted into a video signal. The system of the video signal is, e.g.,NTSC (National Television System Committee). The video signal is inputto a display 108 attached to the digital still camera 101 via a displaysignal processing circuit 107. The display 108 is, e.g., a liquidcrystal monitor.

The video signal is supplied to a video output terminal 110 via a videodriver 109. An image picked up by the digital still camera 101 can beoutput to an image apparatus such as a television set via the videooutput terminal 110. This allows the pickup image to be displayed on animage apparatus other than the display 108. A microcomputer 111 controlsthe image pickup device 104, analog amplifier (AMP), A/D converter(A/D), and camera signal processing circuit 105.

To capture an image, an operator presses an operation button such as ashutter button 112. In response to this, the microcomputer 111 controlsa memory controller 113 to write the output signal from the camerasignal processing circuit 105 into a video memory 114 as a flame image.The flame image written in the video memory 114 is compressed on thebasis of a predetermined compression format by a compressing/stretchingcircuit 115. The compressed image is recorded, via a card interface 116,on the memory card 61 inserted in the card slot.

To reproduce a recorded image, an image recorded on the memory card 61is read out via the card interface 116, stretched by thecompressing/stretching circuit 115, and written into the video memory114. The written image is input to the video signal processing circuit106 and displayed on the display 108 or another image apparatus in thesame manner as when image is monitored.

In this arrangement, mounted on the circuit board 100 are the card slot102, image pickup device 104, analog amplifier (AMP), A/D converter(A/D), camera signal processing circuit 105, video signal processingcircuit 106, display signal processing circuit 107, video driver 109,microcomputer 111, memory controller 113, video memory 114,compressing/stretching circuit 115, and card interface 116.

The card slot 102 need not be mounted on the circuit board 100, and canalso be connected to the circuit board 100 by a connector cable or thelike.

A power circuit 117 is also mounted on the circuit board 100. The powercircuit 117 receives power from an external power source or battery andgenerates an internal power source voltage used inside the digital stillcamera 101. For example, a DC-DC converter can be used as the powercircuit 117. The internal power source voltage is supplied to therespective circuits described above, and to a strobe 118 and the display108.

As described above, the electric card according to this embodiment canbe used in portable electric devices such as the digital still cameraexplained above. However, the electric card can also be used in variousapparatus such as shown in FIGS. 17A to 17J, as well as in portableelectric devices. That is, the electric card can also be used in a videocamera shown in FIG. 17A, a television set shown in FIG. 17B, an audioapparatus shown in FIG. 17C, a game apparatus shown in FIG. 17D, anelectric musical instrument shown in FIG. 17E, a cell phone shown inFIG. 17F, a personal computer shown in FIG. 17G, a personal digitalassistant (PDA) shown in FIG. 17H, a voice recorder shown in FIG. 17I,and a PC card shown in FIG. 17J.

This invention is not limited to the above-described embodiment. It willbe understood by those skilled in the art that various changes in formand detail may be made without departing from the spirit, scope, andteaching of the invention.

1. A non-volatile semiconductor memory device comprising: a NAND string,in which a plurality of memory cells are connected in series and firstand second select gate transistors are disposed on the both ends forcoupling them to a bit line and a source line, respectively; a pluralityof word lines coupled to the respective control gates of the memorycells; and first and second select gate lines coupled to the gates ofthe first and second select gate transistors, respectively, wherein adata read mode is defined by the following bias condition: a selectedword line is applied with a read voltage; one adjacent to the selectedword line within first unselected word lines disposed on the firstselect gate line side of the selected word line is applied with a firstread pass voltage while the others are applied with a second read passvoltage lower than the first read pass voltage; and second unselectedword lines disposed on the second select gate line side of the selectedword line are applied with a third read pass voltage higher than thefirst read voltage.
 2. The non-volatile semiconductor memory deviceaccording to claim 1, wherein in case the selected word line is onenearest to the first select gate line in the data read mode, the wholeunselected word lines are applied with the third read pass voltage. 3.The non-volatile semiconductor memory device according to claim 1,wherein in case the selected word line is one nearest to the secondselect gate line in the data read mode, one adjacent to the selectedword line within the first unselected word lines is applied with thefirst read pass voltage while the others are applied with the secondread pass voltage.
 4. The non-volatile semiconductor memory deviceaccording to claim 1, wherein the NAND string further includes a firstdummy cell disposed between the first select gate transistor and theneighboring memory cell; and a second dummy cell disposed between thesecond select gate transistor and the neighboring memory cell, thecontrol gates of the first and second dummy cells being coupled to firstand second dummy word lines, respectively, and wherein in the data readmode, the first dummy word line is applied with the second read passvoltage while the second dummy word line is applied with the third readpass voltage.
 5. The non-volatile semiconductor memory device accordingto claim 1, wherein the NAND string further includes a first dummy celldisposed between the first select gate transistor and the neighboringmemory cell; and a second dummy cell disposed between the second selectgate transistor and the neighboring memory cell, the control gates ofthe first and second dummy cells being coupled to first and second dummyword lines, respectively, and wherein in the data read mode, the firstdummy word line is applied with a fourth read pass voltage lower thanthe second read pass voltage while the second dummy word line is appliedwith the first read pass voltage.
 6. The non-volatile semiconductormemory device according to claim 1, wherein the data read mode is averify-read mode in a data write sequence.
 7. A non-volatilesemiconductor memory device comprising: a NAND string, in which aplurality of memory cells are connected in series and first and secondselect gate transistors are disposed on the both ends for coupling themto a bit line and a source line, respectively; a plurality of word linescoupled to the respective control gates of the memory cells; and firstand second select gate lines coupled to the gates of the first andsecond select gate transistors, respectively, wherein a data read modeis defined by the following bias condition: a selected word line isapplied with a read voltage; one adjacent to the selected word linewithin first unselected word lines disposed on the first select gateline side of the selected word line is applied with a first read passvoltage while the others are applied with a second read pass voltagelower than the first read pass voltage; and second unselected word linesdisposed on the second select gate line side of the selected word lineare applied with the first read pass voltage.
 8. The non-volatilesemiconductor memory device according to claim 7, wherein the NANDstring further includes a first dummy cell disposed between the firstselect gate transistor and the neighboring memory cell; and a seconddummy cell disposed between the second select gate transistor and theneighboring memory cell, the control gates of the first and second dummycells being coupled to first and second dummy word lines, respectively,and wherein in the data read mode, the first dummy word line is appliedwith the second read pass voltage while the second dummy word line isapplied with the first read pass voltage.
 9. The non-volatilesemiconductor memory device according to claim 7, wherein the NANDstring further includes a first dummy cell disposed between the firstselect gate transistor and the neighboring memory cell; and a seconddummy cell disposed between the second select gate transistor and theneighboring memory cell, the control gates of the first and second dummycells being coupled to first and second dummy word lines, respectively,and wherein in the data read mode, the first dummy word line is appliedwith a fourth read pass voltage lower than the second read voltage whilethe second dummy word line is applied with the first read pass voltage.10. The non-volatile semiconductor memory device according to claim 7,wherein the data read mode is a verify-read mode in a data writesequence.
 11. A non-volatile semiconductor memory device comprising: aNAND string, in which a plurality of memory cells are connected inseries and first and second select gate transistors are disposed on theboth ends for coupling them to a bit line and a source line,respectively; a plurality of word lines coupled to the respectivecontrol gates of the memory cells; and first and second select gatelines coupled to the gates of the first and second select gatetransistors, respectively, wherein a data read mode is defined by thefollowing bias condition: a selected word line is applied with a readvoltage; one adjacent to the selected word line within first unselectedword lines disposed on the first select gate line side of the selectedword line is applied with a first read pass voltage while the others areapplied with a second read pass voltage lower than the first read passvoltage; and one adjacent to the selected word line within secondunselected word lines disposed on the second select gate line side ofthe selected word line are applied with third read pass voltage higherthan the first read pass voltage while the others are applied with thefirst read pass voltage.
 12. The non-volatile semiconductor memorydevice according to claim 11, wherein the NAND string further includes afirst dummy cell disposed between the first select gate transistor andthe neighboring memory cell; and a second dummy cell disposed betweenthe second select gate transistor and the neighboring memory cell, thecontrol gates of the first and second dummy cells being coupled to firstand second dummy word lines, respectively, and wherein in the data readmode, the first dummy word line is applied with the second read passvoltage while the second dummy word line is applied with the first readpass voltage.
 13. The non-volatile semiconductor memory device accordingto claim 11, wherein the NAND string further includes a first dummy celldisposed between the first select gate transistor and the neighboringmemory cell; and a second dummy cell disposed between the second selectgate transistor and the neighboring memory cell, the control gates ofthe first and second dummy cells being coupled to first and second dummyword lines, respectively, and wherein in the data read mode, the firstdummy word line is applied with a fourth read pass voltage lower thanthe second read voltage while the second dummy word line is applied withthe first read pass voltage.
 14. The non-volatile semiconductor memorydevice according to claim 11, wherein the data read mode is averify-read mode in a data write sequence.